Recently, inkjet printers has been rapidly diffused for its advantageous properties such as low noise and low running cost, and color printers which is printable on the regular paper has been actively provided to the market. However, satisfying all the properties required such as color reproducibility, endurance, light stability, drying property, feathering and color bleed of images, duplex printing, discharge stability, and the like is very difficult for those inkjet printers, the inks which correspond to each preferred property are being selectively used instead.
The inks for use in such inkjet recording in general consist mainly of water and contain colorant (e.g. dye, pigment) and moistening agent such as glycerin, and the like for anticlogging. Dyes, for its excellent color-producing property and stability, are used as colorant. However, light stability and waterproofness, etc. of the images produced by dye's inks are not satisfactory. Waterproofness may be improved somehow by upgrading the inkjet recording paper containing ink absorber layer, however, it is unsatisfactory for the regular paper.
Recently, in order to improve these problems a recording ink, of which a pigment such as organic pigment, carbon black, and the like instead of dyes is reduced to particles using surfactants or dispersants and dispersed in a medium such as water, is being investigated for use as a colorant. For example, the process for improving discharge stability in which an organic pigment is reduced to particles of 50 nm or less in diameter using specific dispersant, is disclosed in Patent Literatures 1 and 2. And an ink for inkjet printing using specific organic pigment and anionic dispersant is disclosed in Patent Literature 3.
However, using any dispersant does not help atomizing smaller than primary particle of the organic pigment and making diameter of the primary particle of the organic pigment 50 nm or less without degrading color tone is almost impossible. Also, there are problems in that when a low-molecular dispersant is used, scratch resistance and waterproofness are low, and that when a high-molecular dispersant is used, scratch resistance and waterproofness are insufficient.
Because inkjet recording requires a stable discharge of ink droplets from a microscopic nozzle of the inkjet recording head, ink solidification caused by drying of orifice of the inkjet recording head should be avoided. However, when an ink in which the above-mentioned organic pigment is dispersed is used for the inkjet recording, clogging or no discharge of the ink may occur. Especially when printing is suspended for a long period, the clogging of nozzle, etc. are likely to occur, and the thickened ink may be deposited in the maintenance mechanism such as inside the nozzle caps or suction tubes, hindering functions of the maintenance mechanism. Furthermore, when printing was temporarily suspended, or the nozzle corresponding to the space of printing documents or images having spaces was suspended for a while, there have been a lot of problems such as printing failures (intermittent discharge failures) caused by distorted injection direction of ink droplets.
The other dispersion method include, for example, the surface reforming recording ink by which the surface of the pigment particles may be stably dispersed without pigment dispersant by modifying it with carboxyl group, carbonyl group, sulfone group and hydroxyl group, and the like and giving hydrophilic property. The surface reforming carbon black, a black recording ink by which the pigment particles can be stably dispersed without pigment dispersant by introducing hydrophilic groups on the surface of carbon, has been developed. The surface reforming color pigment, a color pigment ink by which the pigment particles can be stably dispersed without pigment dispersant has also been developed. However, scratch resistance and waterproofness of these surface reforming recording inks on the regular paper or exclusive gloss paper are not sufficient.
To solve these problems, an inkjet ink utilizing the dispersing element of micro-capsule or emulsion type by which the pigment particles are coated with resin is proposed. In this proposal, since the pigment particles are coated solidly by resin, dispersion may be stable for a long period and the discharge stability may be improved somewhat, however, it is difficult to reduce the dispersion particle diameter to 100 nm or less even coating with a pigment of the primary particle diameter of 50 nm to 100 nm.
An inkjet ink is proposed in Patent Literatures 4 and 5 wherein the particle diameter of contained color materials is made into a uniform nanometer size by attaching dyes to the surface of a metal oxide and coating the surface with an ionic group-containing organic compound. By this proposal, waterproofness is improved somewhat compared to the ink utilizing dyes; however, light stability is inferior to the dye ink therefore not appropriate for actual use.
If a dispersant is used for dispersion of above pigment, or if the surface of the pigment is made hydrophilic, a method in which a polymer is added to the ink for making up insufficient waterproofness and scratch resistance may be used, however, it is impossible to obtain sufficient waterproofness with soluble polymer. For this reason, a method of using a water dispersible polymer particle is proposed. However, if the water dispersible polymer particle remains in a form of a particle on the ink record after printing, the particle may easily be re-dispersed in water and therefore waterproofness is not sufficient.
Recently, a composite pigment by which an organic pigment is coated around the inorganic pigment particle is proposed. If the inorganic pigment particle of around 5 nm to 50 nm is utilized as a core material, it is possible to obtain a coloring pigment of less than 100 nm even when being coated with an organic pigment. The composite pigment color material of 10 nm to 20 nm has been proposed and practically applied as disclosed in Patent Literatures 6 and 7. And applying these composite pigment particles to paints or inkjet inks are also proposed as disclosed in Patent Literatures 8 and 9. However, in these proposals, it is difficult to ensure the dispersion stability of pigment, especially the dispersion stability under preservation for a long period. Further, since resins for fixing are not contained, waterproofness and scratch resistance of ink records are insufficient.
Examples of ink composition containing polymers include an ink in which pigment and polymer emulsion are dispersed in water as outlined in Patent Literature 10, an ink in which a pigment is dispersed in polymer emulsion/dispersion of water-insoluble resin as outlined in Patent Literature 11 and an ink using a polymer emulsion having specified film-forming temperature as outlined in Patent Literature 12.
To improve fixing ability of composite pigment particles, addition of high-molecular dispersant is proposed in many literatures (e.g. Patent Literatures 13, 14, 15, 16, 17, 18, 19 and 20). In these proposals, approaches are done to give a dispersant a fixing function by using high-molecular dispersant, however, because the high-molecular dispersant proposed above is highly water-soluble, sufficient waterproofness cannot be obtained although scratch resistance may be in sufficient level.
Furthermore, an ink containing emulsion resin for improving waterproofness of ink records utilizing these composite pigment particles is proposed in Patent Literature 21. However, because this emulsion resin is water dispersible, it becomes re-dispersed when ink records get wet; therefore cannot prevent the outflow of pigment particles.
Thus, in the present situation, it has been desired that a recording ink which excels in discharge stability and preservation stability, which has good image retention capabilities such as scratch resistance, waterproofness and light stability, etc., which makes it possible to obtain a high quality image that excels in color reproducibility of color image, and which is suitable for inkjet recording; and related techniques are provided as soon as possible.    Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 09-263720    Patent Literature 2 JP-A No. 09-263722    Patent Literature 3 JP-A No. 2002-088286    Patent Literature 4 JP-A No. 11-166127    Patent Literature 5 JP-A No. 2001-192582    Patent Literature 6 JP-A No. 2002-146231    Patent Literature 7 JP-A No. 2002-161221    Patent Literature 8 JP-A No. 2003-049096    Patent Literature 9 JP-A No. 2003-055591    Patent Literature 10 Japanese Patent Application Publication (JP-B) No. 62-001426    Patent Literature 11 JP-A No. 55-157668    Patent Literature 12 JP-A No. 01-217088    Patent Literature 13 JP-A No. 2003-049096    Patent Literature 14 JP-A No. 2003-055591    Patent Literature 15 JP-A No. 2003-105229    Patent Literature 16 JP-A No. 2003-171594    Patent Literature 17 JP-A No. 2003-192938    Patent Literature 18 JP-A No. 2003-327866    Patent Literature 19 JP-A No. 2003-268278    Patent Literature 20 JP-A No. 2003-327880    Patent Literature 21 JP-A No. 2003-049097